Frederick makes a habit of removing rocks and other obstacles so Chrom doesn't trip over them.
However he neglects the pebble that Chrom trips over in his C support with Sumia. If Frederick was more diligent in his duties and properly removed the pebble, Chrom would have had no reason to converse with Sumia. Therefore, their relationship would have never developed.
Likewise, it could be said that Frederick started removing pebbles with the intention of PREVENTING Chrom's support with Sumia. However, he didn't think Sumia would be so devious and to plant a pebble in the middle of a room just for Chrom to trip on, so she could get her hands on him.
Don't even get me started on why she sent Shepard imposters to Regna Ferox and disposed of a fellow Pegasus Knight and injured their Pegasus just so she could impress and rescue Chrom. Or how she purposely breaks things at camp so Cordelia can fix them and not have any free time to spend with Chrom.
Official Miriel of the Fire Emblem Awakening Boards.